In principle, switched-mode power converters function by alternately connecting and disconnecting the source to the load by means of an active switch. Therefore, in order to deliver continuous power to the load, some means of intermediate energy storage must be included within the converter to provide power during the interval when the source is disconnected. Since the amount of energy delivered to the load by these elements is proportional to the length of the conducting portion of the switching interval, increasing the operating frequency of the converter (i.e. reducing the length of the conducting and switching intervals) also reduces the amount of internal energy storage required. Since the size and weight of switched-mode power converters is most often dominated by these internal energy storage elements, increasing the operating frequency of switched-mode converters is often employed as a technique to reduce their size. However, as the operating frequency of these converters is increased, frequency related losses also increase, ultimately limiting the maximum practical operating frequency.
At low operating frequencies, power dissipation in the active switch(es) of a conventional switched-mode power converter are primarily dominated by conduction losses. However, as the operating frequency of the power switch(es) is increased another type of power loss, commonly called switching loss, begins to constitute a significant portion of the total power loss in the switch(es). Switching losses result from dissipation of energy due to the simultaneous occurrence of voltage across and current through a switch during its switching transitions. Since a finite amount of energy is lost during each switch transition due to this overlap, the power dissipated in the switch(es) of a switched-mode power converter is proportional to its operating frequency. This makes reduction of switching loss a key factor in the improvement of the operating efficiency of switched-mode inverters and DC-to-DC converters operating at high frequencies.
This switching loss reduction is achievable through the use of quasi or fully resonant inverters and converters which employ a controlled waveform switching operation that reduces or eliminates the simultaneous occurrence of voltage across and current through the power switch(es) during switching transitions. An illustrative example of a quasi resonant converter is disclosed in U.S. Pat. No. 4,415,959 and an illustrative example of a fully resonant converter is disclosed in U.S. Pat. no. 4,607,323.
Previous investigators have concentrated primarily on circuit design techniques which reduce or eliminate switching losses by control of the waveforms applied to the power switch. The inverter circuit of the fully resonant converter relies on the shaping of the voltage waveform across the power switching device to achieve the low switching loss that permits operation at high frequencies. One of these approaches, [U.S. Pat. No. 4,607,323], which applies Class-E amplifier techniques to a DC-to-DC converter to produce a zero-voltage-switching converter has relied heavily on the use of a shunt diode in parallel with the power switch or the body diode of the semiconductor power switch. This diode which shunts the main conduction path of that switch is properly oriented or polarized in order to prevent negative or reverse current flow through the power switch by clamping negative going voltages. This diode acts to provide a significant region of effective load impedance on the inverter section over which there is zero switching loss and increases the range of load on the entire converter over which operation of the power switch without switching loss can be maintained.
Elimination of switching losses in the zero-voltage-switching DC-to-DC converter is achieved according to U.S. Pat. No. 4,607,323, by designing a load network including the rectifier circuit that properly loads the inverter circuit with a range of impedances that achieve the desired switching waveform control. Hence the power switching devices are operated with substantially zero switching loss. Proper wave shaping across the power switch occurs only over a relatively narrow range of inverter load impedance. As the resistive component of this load impedance is decreased switching losses generally decline at the expense of increased conduction losses. Conversely, as the resistive component of this load impedance is increased switching losses generally increase quickly even though conduction losses are declining. The power switch in this example is shunted by a diode. Recognition of the operative effect of this diode in parallel with the power switch permits the converter to operate over a wider range of input voltage and output power without switching loss or with very low switching loss. Conduction loss in the power switch(es) was assumed to be negligible in this U.S. Pat. No. 4,607,323.
The design techniques disclosed in U.S. Pat. No. 4,607,323, are concerned solely with elimination of switching losses in the main power switch. At sufficiently low frequencies where conduction losses are readily kept low this approach has merit. However, at high frequencies this approach generally suffers from excessive conduction losses. Unfortunately, the shunt diode provides zero switching loss at the low end of the load resistance range where conduction losses are largest. In particular, switching loss is eliminated with these techniques by operating the inverter into a load impedance with a resistive component low enough to ensure conduction of the diode in shunt with the active power switch over the operating line and load range of the converter. However, this elimination of switching losses is achieved as indicated above at the expense of increased conduction losses in the converter since operation of this inverter with low resistive loads increases currents in the circuit, thereby increasing the conduction losses.
Conduction loss in the switching device was assumed to be negligible in the zero-voltage-switching power converters of the prior art which have been optimized to reduce only switching loss. However, in a practical zero-voltage-switching power converter, realistic power switching devices do not have negligible conduction losses particularly at high frequencies where the size of a suitable power switch is limited by its parastic capacitance. To be useful in a practical high frequency power converter, the total loss in the inverter must be minimized, including both switching losses and conduction losses, and must remain low over the wide line and load range commonly encountered in the normal operation of such a converter.
Parasitic capacitances that are normally part of a switching device in a high frequency power converter tend to increase switching losses by discharging in each switching cycle. Hence operation of the converter must be carefully controlled to achieve zero voltage switching and thereby to minimize these switching losses. This requires consideration of the effect of the parasitic capacitance on circuit operation. In addition, switching device resistance causes a resistive power loss within the switching device. These two characteristics namely capacitance and resistance of the semiconductor switching device (particularly a MOSFET device) are each dependent on its physical size. Increasing the size of the semiconductor switching device increases its capacitance and decreases device resistance. Capacitance can be reduced by reducing device size but this also increases device resistance. Hence a desire to lower switching losses by reducing the switching device size to reduce parasitic capacitances also increases conduction losses. This situation is further complicated in zero-voltage-switching resonant converters where, the parasitic capacitance of the semiconductor switching device is an integral part of the resonant circuit making the value of this capacitance important for proper circuit operation.
From the above discussion it should be clear that conduction losses and switching losses in the semiconductor switching device are interrelated and that proper selection of the device as well as choosing the proper operating region for the inverter are both necessary in order to minimize the total loss in the power switch(es) of a practical switched-mode power converter.